The invention relates to directionally dependent control of the power for the downlink in a cellular radio communication network with adaptive antennas.
In radio communication systems, information (for example speech, image information or other data) is transmitted with the aid of electromagnetic waves via an air interface between transmitting and receiving stations (base station and subscriber station, respectively). The emission of the electromagnetic waves is performed in this case with the aid of carrier frequencies that are in the frequency band provided on the respective system. In the case of the GSM (Global System for Mobile Communication), the carrier frequencies are at 900, 1800 and 1900 MHz. Frequencies in the frequency band of approximately 2000 MHz are provided for future mobile radio systems using CDMA or TD/CDMA transmission methods via the radio interface, for example the UMTS (Universal Mobile Telecommunication System) or other 3rd generation systems.
The data transmission takes place via frames in the case of these radio communication systems. A division of a broadband frequency domain into a plurality of time slots of equal time duration is provided in the case of a TDMA component (TDMA: Time Division Multiple Access). The time slots are used partially in the downlink DL (downlink from base station to subscriber station), and partially in the uplink UL (uplink from subscriber station to base station). One or more switching points are situated therebetween. The same is repeated for further carrier frequencies. Information of a plurality of connections is transmitted in radio blocks within the time slots. Radio blocks for user data transmission currently include sections with data in which training sequences or midambles known at the receiving end are embedded.
The switching points can be defined synchronously in all cells of the radio communication system. In this case, a time slot in the overall radio communication system is used exclusively in the uplink UL or exclusively in the downlink DL. Additional flexibility is achieved by defining the switching points asynchronously. In this case, some cells of the radio communication system use one time slot for UL and others for DL.
Because the distance between transmitter and receiver frequently fluctuates strongly during operation, it is desired to match the transmit power over up to a plurality of orders of magnitude, in order to keep the ratio of energy per bit/noise-power density or the ratio of signal/interferer or carrier power/ interference power in the limit or target range. On the one hand, the receive power must be at a minimum level that is required for the desired surface quality, but on the other hand as little interference as possible is to be produced.
DE 198 03 188 discloses a method and a base station for, in particular, TDMA/CDMA transmission methods (CDMA: Code Division Multiple Access), in the case of which the signals transmitted from the base station in the downlink are specifically amplified in the direction of the assigned subscriber station, and attenuated in the other directions. For this purpose, spatial covariance matrices are estimated in the base station for each subscriber station in order to determine amplifying interference from the signal received in the uplink, and thereafter a beam-shaping vector is calculated which maximizes the signal/noise ratio at the receiver. A general eigenvalue problem is solved in this case without iteration. Thereupon, transmitted signals are weighted with the beam shaping vector for the corresponding radio link and fed to the antenna element of the antenna arrangement for emission. The covariance matrix is determined from a priori assumptions with the aid of a mathematical model. The base station measures nothing during the transmission to its subscriber station in the corresponding downlink time slot. Consequently, uplink measurements of the training sequences must be used for estimation, in order to estimate the downlink covariance matrix.
In other words, in the case of this method the antenna gain of the antenna arrangement of the base station is maximized in specific directions, which are assigned to the dedicated subscriber stations, by appropriately driving the individual antenna elements of the antenna arrangement. That is to say, the power that is transmitted from an antenna group to an assigned subscriber station is emitted in a maximized fashion by constructive interference in the direction in which this subscriber station is located.
These radio communication systems have a cellular structure in which in each case a base station with at least one transmitting antenna arrangement supplies subscriber stations in a specific radio cell zone. In this case, disturbing interference with subscriber stations of adjacent radio cell zones can arise that are supplied from a neighboring base station. This is the case, in particular, when the base station transmits to a subscriber station assigned to it in a time slot in which the subscriber station in the adjacent radio cell zone receives data from its base station, the adjacent one.